The present invention relates to an inspection apparatus using magnetic resonance, and more particularly, to an apparatus for acquiring images of an object having periodic movement such as heartbeat, respiration.
A magnetic resonance imaging (MRI) apparatus is a medical image diagnostic apparatus for magnetically resonating water components in an arbitrary plane which traversing an object of inspection and acquiring a slice image of the plane from generated magnetic resonance signals. Generally, when a slice gradient magnetic field for specifying a plane to obtain a slice image is applied and simultaneously an excitation pulse for exciting magnetization in the plane is applied, magnetic resonance signals (echoes) generated at a stage when phases of excited magnetization are coincided are obtained. In order to give position information to the echo, a phase-encoding gradient magnetic field and a readout gradient magnetic field are applied in a period from the excitation to the acquisition of echoes. The measured echo is called "raw data" and stored in a memory called a k-space having the axis of abscissa in the readout direction and the axis of ordinate in the phase-encoding gradient magnetic field direction. The raw data is subjected to inverse Fourier transformation, thereby reconstructing an image.
The pulses and the gradient magnetic fields for generating the echoes are applied on the basis of a preset pulse sequence. Various pulse sequences are known according to purposes. For example, in a gradient echo imaging as a general imaging, the pulse sequence is repeatedly operated, the magnitude of the phase-encoding gradient magnetic field (phase-encoding quantity) is sequentially varied every repetition, and the echoes of the number necessary to acquire one slice image are sequentially measured. At least tens m/sec is necessary as the time interval to repeat the pulse sequence in order to wait recovery of the magnetization. The image acquisition time per image is few seconds or longer since the number of echoes necessary to reconstruct one image is usually 256. Consequently, the gradient echo imaging is not suited for an image acquisition of an object having movements.
On the contrary, there is a retrospective gating for continuously acquiring images of an object having movement such as the heart. According to the method, a phase-encoding quantity (p) is fixed and a pulse sequence is repeated a predetermined number (S) of times with predetermined repetitive times (TR), thereby measuring (S) echoes. The (S) echoes measured are used to reconstruct images at different time phases. The measurement is repeated while sequentially varying the phase-encoding quantity and echoes necessary to reconstruct a plurality of images are measured. In case of measuring (N) echoes for one image, the pulse-encoding quantity (p) usually varies as -N/2, . . . , 0, . . . , (N/2-1)
In parallel with the measurement of the echoes, information of movement of the object is recorded. When an image is reconstructed, echoes are rearranged on the basis of the information. For example, when the heart is the object of image acquisition, echoes are rearranged by using an electrocardiogram recorded simultaneously with image acquisition. The relation between the electrocardiogram (ECG) and the phase-encoding quantity is, for example, as shown in FIG. 2. In the diagram, an example when S=8 is shown. In case of continuously acquiring images of (M) frames per heartbeat (M=5 in FIG. 2), in order to reconstruct an image at the (j)th time phase (j=1, . . . , M), echoes measured at time phases nearest to the time phase (j) among echoes with the phase-encoding quantities (p) are collected with respect to all of the phase-encoding quantities (p). Table 1 shows the result when the echoes are rearranged according to the time phase and the phase-encoding quantity.
TABLE 1 ______________________________________ p 1 2 3 ______________________________________ time phase 1 1, 6 3, 7 -- time phase 2 2, 7 4, 8 -- time phase 3 3, 8 1, 2 time phase 4 4 1, 5 3 time phase 5 5 2, 6 4 ______________________________________
Numbers in Table 1 are measurement numbers. For example, at the time phase 1, echoes of measurement numbers 1 and 6 are arranged in the phase-encoding quantity 1. The pair of echoes is subjected to a two-dimensional inverse Fourier transformation and an image of the heart at the time phase (j) can be consequently acquired. By reconstructing images with respect to all of the time phases (j), continuous images of the heart can be acquired. The details of the retrospective gating are described by Generald W. Lenz et al., "Retrospective Cardiac Gating: A Review of Technical Aspects and Future Directions" Magnetic Resonance Imaging, Vol. 7, pp. 445-455, 1989.
According to the retrospective gating, since the movement of the object of image acquisition is not always constant, omission of echo measurement sometimes occurs. For example, in the measurement shown in FIG. 2, an echo when p=2 at the time phase 3 is missing. Since the number (S) of echo measuring times is fixed with respect to each (p), the probability of omission occurrence of echo measurement is equal with respect to any phase encoding quantity.
However, an influence exerted on an image by the omission occurrence of echo measurement is not uniform with respect to (p) in which the omission of echo measurement occurs. The more the phase-encoding quantity is close to zero, the larger the power of the echo generally is. If the omission of echo measurement occurs when the phase-encoding quantity is close to zero, there is a problem that a large amount of an artifact occurs in an image.